A delta wave is a slow brain rhythm most prominent during deep, slow-wave sleep and generally measuring about 0.5–4 hertz.
Delta waves are high-amplitude, low-frequency oscillations recorded with EEG. They become especially prominent during the deepest stages of non-rapid-eye-movement sleep, now commonly called N3 sleep. Their appearance reflects synchronized activity across large populations of cortical neurons, alternating between relatively active and relatively quiet states.
The frequency range is a convention rather than an absolute biological boundary. Different references may use slightly different limits, and EEG rhythms can overlap. Delta activity can also occur during anesthesia or certain brain disorders, so seeing delta waves does not by itself prove that a person is naturally asleep.
Sleep researchers distinguish delta activity from sleep spindles, which are faster bursts around 11–16 hertz associated with N2 sleep. Alpha activity is more typical of relaxed wakefulness, while beta activity is faster and often associated with alert mental activity. EEG interpretation depends on age, state, electrode location, and clinical context.